U.S. Pat. No. 6,233,045 B1 discloses a self-mixing interference sensor for determining a velocity of an object. The self-mixing sensor comprises a laser emitting a laser beam being directed to, for example, the ground. The laser beam is reflected by the ground and the reflected laser beam enters the cavity of the laser. The reflected laser beam, entering the laser cavity, interferes with the present electromagnetic field. The interference, the so called self-mixing interference, appears as intensity variations of the electromagnetic field within the cavity. The main frequency of the self-mixing interference signal is the Doppler frequency, wherein the Doppler frequency is used to determine the velocity of the object.
This determination of the velocity can be disturbed by, for example, harsh automotive environments and tough road conditions, if the object is, for instance, a vehicle. The laser beam may also temporally be blocked by scattering elements between the laser and the road surface, i.e. the ground, or the self-mixing feedback, i.e. the reflected laser beam, may become too weak due to too large defocus or presence of blocking objects like leaves or other rubbish which may be lifted by air turbulence. The quality of determining the velocity can therefore be reduced.